Erbium doped fiber amplifiers (EDFAs) are currently the most commonly used optical amplification devices to amplify weak optical signals, weakening of the optical signals being caused for example by attenuation in transmission fibers. EDFAs are largely used in both short and long haul optical communication networks, as well as in CATV broadcast networks. The newest wavelength division multiplexing (WDM) systems require even more use of EDFAs as multiple wavelength light supporting plural channels are carried simultaneously by the same fiber. Thus the amplification gain of the EDFAs must be shared among the different wavelengths of light.
A conventional EDFA has a usable bandwidth of about 35 nm. Assuming a WDM channel spacing of 50 GHz (0.4 nm), this bandwidth would allow up to 80 channels to share the gain of the amplifier used to compensate for losses resulting from signal propagation in the optical fiber. Although the channel spacing can be narrowed to increase the number of transmitted channels, this can complicate signal detection systems (receivers) mainly due to channel crosstalk. It also makes manufacturing of WDM passive components such as dense wavelength division multiplexers/demultiplexers difficult. Very narrow channel spacing causes nonlinear effects such as four photon mixing in the transmitting fiber, which degrades system performance.
It is known that conventional EDFAs could amplify longer wavelength light such as around 1585 nm through use of a long Erbium doped fiber and by employing 1530 nm pumping. However, the efficiency is very low due to the required two-step pumping process (980 to 1530 nm, than 1530 to 1585 nm). To obtain a reasonable amplification gain, large amounts of pump power, such as is obtained by using a MOPA (Master Oscillator Power Amplifier) laser, or a combination of 4 or more 980 nm and/or 1480 nm pump lasers, has to be used. It has been reported that a 20 dB gain can be obtained using a total of 500 mW pump power, with a difference of gain of more than 10 dB between the two amplification bands (1545 nm and 1585 nm). Circulators together with reflective gratings were used to combine shorter and longer wavelength bands. These configurations either require expensive and short lifetime components, or by using many active pump lasers, systems require complex electronic surveillance circuitry and have the potential of a high failure rate.